The present invention relates generally to high-quality image formation apparatus, and more particularly to such an image formation apparatus for performing a gradation recording by changing the size of dots on the basis of inputted image data.
As an output terminal of a personal computer, a workstation and others, there is known a laser beam printer (which will be referred hereinafter to as LBP) which is based on an electrophotographic technique and a laser technique. Recently, the LBP is widely and increasingly being used because of having great advantages in recording speed and printing quality. Generally, the image output apparatus such as LBP based upon the electrophotographic process tends to be difficult to output a half-tone image because the electrophotographic process has a problem in stability, thus outputting a two-gradation image (black and white color image). For obtaining a half-tone image by such a two-gradation printer, one known technique involves performing the binary-coding of the image pixels in accordance with the dither method. The principle of the dither method will be described hereinbelow with reference to FIGS. 11 and 12. In FIG. 11, the pixels constituting the image are grouped into a plurality of blocks each comprising N pixels.times.M pixels (4 pixels.times.4 pixels in the illustration). The level of each of the block pixels is compared with each of N.times.M matrix thresholds so as to perform the binary-coding in accordance with the comparison results. That is, the pixel is set to "1" when the pixel level is higher than the threshold and set to "0" when being lower than the threshold. A dither image can be obtained by repeatedly performing this binary-coding operation at every each matrix. FIG. 12 shows a circuit arrangement for this binary-coding dither method. In FIG. 12, a comparator is coupled to an input line In and a threshold matrix memory Mx so that an input image signal inputted through the input line In are successively shifted at every pixel and the row and column of the threshold matrix memory Mx are addressed in correspondence with the shifting of the input image signal to successively read out the thresholds to compare the threshold with the input image signal at every pixel, whereby a binary-coded recording image signal is outputted through an output line Out to a print engine (not shown) for printing.
However, not only does the binary-coding dither method cause moire pattern and texture to be generated in the printed image, but also makes it difficult to perform the density adjustment of the image because of the binary-coding of the image data in density. Moreover, in the case that character data are mixed with half-tone picture data, difficulty is encountered to prevent the character quality from being deteriorated because the character is also dithered.